Mechanical properties of SiC/Ti-15V-3Cr-3Sn-3Al composites by spark plasma sintering

The pioneering work related to the spark plasma sintering (SPS) technique, also known as pulsed electric current sintering (PECS), started in around 1906 when the first direct current (DC) resistance sintering (RS) apparatus was developed. Later, a similar process was developed and patented in the 1960s. The present-day SPS which is now widely used for sintering metals and ceramics was introduced by the Sumitomo Coal Mining Co., Ltd. of Japan in 1990. Since then this technique which is based on the idea of using plasma generated by an electric discharge machine for sintering has attracted immense attention in the area of powder technology and development of the composites. The SPS process can be considered as a modified hot pressing process where a pulsed electric current is passed through a graphite die and the specimen is heated by the Joule heat transferred from the pressing die. In SPS, the compaction and sintering stages are combined in a single operation. Due to the pulsed electric current and the spark plasma effect, the SPS process is capable of introducing simultaneous rapid heating and cooling rates; accompanied with high pressure, this process can realize nearly full densification at a relatively lower temperature, within a very short time. In the past, materials were confined only to the monolithic form, but for better physical, chemical and tribological properties, composite materials have evolved. Metal matrix nanocomposites (MMNCs) have evoked keen interest in recent times because of their excellent structural and functional properties and have the potential to replace the existing materials in a wide range of applications. MMNCs refer to materials where rigid nanosized reinforcements, typically having size <100 nm, are embedded in ductile metals or alloys which act as the matrices providing attractive physical and mechanical properties such as high specific modulus, strength-to-weight ratio, fatigue strength, temperature stability and wear resistance. The properties of the MMNCs can be designed and custom-made as per the requirement of the application. MMNCs received much attraction as compared to the metal matrix composites (MMCs) due to the size and strength of the nanometric reinforcements. Apart from the nanofillers, a fine grain size of the matrix could also contribute to the improvement in the properties of the composites. As conventional processing techniques require a long holding time at high sintering temperatures, which can damage the structure of the nanofillers, processes like the SPS are the ideal routes for development of such composites. Also during conventional sintering, abnormal grain coarsening becomes particularly severe which in turn implies that achieving a very high level of densification and maintaining a fine grain size is very difficult. Attaining a uniform dispersion of the nanofillers in the composites is not easy using liquid-processing techniques due to the difference in the densities of the nanofiller, and the matrix and the non-wettability at the interface between them lead to a heterogeneous structure that affects the overall properties of the nanocomposites. By SPS, prevention or reduction in grain growth, to maintain the nanostructure of the matrix, is possible through careful control of consolidation parameters, particularly heating rate, sintering temperature and time. Due to the short period of sintering, grain growth can be restricted, and materials having submicron-sized or nanosized microstructures having enhanced properties can be developed. As the SPS technique requires very short sintering time, it is ideally suited for the development of nanocomposites reinforced with carbonaceous nanofillers like graphene or its derivatives and carbon nanotubes (CNTs), as short sintering time is essential for preserving their structural integrity and intrinsic properties. A wide variety of materials like metals and alloys, ceramics, composites, cermets etc. can be successfully developed by the SPS process. However, it should be noted that although SPS presents many advantages as compared to other conventional sintering techniques, it also has a few limitations. One of the major drawbacks of the SPS process is the heterogeneity of temperature field during the temperature cycle, resulting in heterogeneous microstructures in the sintered samples. Also, SPS allows only simple symmetrical shapes and is expensive as it requires a pulsed DC generator. The focus of this chapter will be on the fundamentals of the SPS technique, the kinetics of densification and grain growth during SPS and how the SPS technique could be effectively used to develop metal matrix nanocomposites (MMNCs). The chapter provides a detailed overview of the SPS process, the current state of research in the area of MMNCs developed by the SPS and its future.

Consolidation of titanium tri-aluminide by spark plasma sintering: K.Kobayashi et al. (National Industrial Research Inst., Nagoya, Japan.) J. Jpn Soc. Powder Powder Metall., vol 44, no 6, 1997, 554–559. (In Japanese.)

Microstructures of binderless tungsten carbides sintered by spark plasma sintering process

Spark plasma sintering of a lunar regolith simulant: effects of parameters on microstructure evolution, phase transformation, and mechanical properties

Electrocaloric effects in spark plasma sintered Ba0.7Sr0.3TiO3-based ceramics: Effects of domain sizes and phase constitution

Chapter 11.2.3 - Spark Plasma Sintering (SPS) Method, Systems, and Applications

Ni-free Ti-based bulk metallic glass with potential for biomedical applications produced by spark plasma sintering

Gas-atomized Ni52:5Nb10Zr15Ti15Pt7:5 metallic glassy alloy powders were consolidated by a spark plasma sintering (SPS) process. The densification behavior during the SPS process as well as the structure, thermal stability and mechanical properties of the sintered specimens were investigated. The glassy alloy powders were densified rapidly when the temperature exceeded about 740 K. The density of the sintered specimens increased with an increase in sintering temperature. The specimens with full densification and no crystallization were obtained by the SPS process at a sintering temperature of 773 K with a loading pressure of 600 MPa. The sintered specimens exhibit high-strength and can meet large-size requirement. [doi:10.2320/matertrans.ME200817]

The effect of sintering process on the microstructure and the mechanical properties of aluminum syntactic foam were investigated in this study. Two different sintering processes of spark plasma sintering and hot pressing were used. Glass hollow spheres with a size of 50–80 μm was used to fabricate the foams having various volume fractions of the spheres in the range of 10–30%. Microstructural analysis revealed that the glass hollow spheres were uniformly distributed in the aluminum matrix, both in the spark plasma sintered and hot pressed ones. As the volume fraction of the spheres increased from 10 to 30%, the density, micro-hardness and compressive strength of the foams were decreased. In comparison to the foams fabricated by hot pressing method, the spark plasma sintered foams had slightly lower density and mechanical strength. In nanoindentation study, it was found that the aluminum matrix in the foam prepared by the spark plasma sintering process had lower strength than foam prepared by the hot pressing process. This is likely because of shorter sintering time used in the spark plasma sintering process than the hot pressing.

Oxides are generally non-flammable, durable and non-toxic materials; high safety and reliability in a battery system is assured by the use of oxide as an electrolyte instead of the highly-reactive non-aqueous liquid. To develop an oxide-based all-solid-state lithium-ion battery (ASS-LIB) for applications such as electric vehicles, the formation of strong interfacial contact between the electrolyte and the electrode powder is desirable, which can be achieved through powder technology. In oxide-based ASS-LIB, the good contact interfaces should be prepared by a simple powder sintering process, and also produces little ion-blocking impurities.[1] A Li2O-Al2O3-TiO2-P2O5 (LATP) solid electrolyte with NASICON-type structure possesses high bulk conductivity of over 10-4 S cm-1 at R.T. and high electrochemical stability (applicable potential range: 2.6−6 V vs Li/Li+)), which is a suitable candidate for assembling ASS-LIB with high safety and chemical stability. However, the poor interfacial contact between electrode and solid electrolyte is one of the general problems for showing the electrochemical activity of ASS-LIB. Spark plasma sintering (SPS) would be a useful tool for designing the ASS-LIB since dense ceramics can be sintered at shorter time with suppressing the formation of by-products at the interface. Actually, Aboulaich et al. have successfully measured the charge-discharge profiles of Li3V2(PO4)3 and LiFePO4 electrodes at ASS-LIB assembled with Ge-based Li1.5Al0.5Ge1.5(PO4)3 electrolyte by this SPS technique.[2] However, most electrode materials produce impurities by contact with the solid electrolytes containing Ti4+ ions as LATP after sintering at high temperature (900oC).[3] In this presentation, the ASS-LIBs using LATP electrolyte were assembled by SPS with tungsten carbide die, instead of conventional carbon die, which could be applied high pressure over 200 MPa and processed at low temperature below 300 °C, resulting in suppressing the formation of by-products at the interface. Samples for AC impedance measurement were prepared by SPS of Au/LATP powder (average particle size: 0.2 μm)/Au. The sintering temperature, applied pressure and time at SPS were 300 °C, 600 MPa and 1 min for the densification. During the SPS process, the shrink of the LATP pellet was observed at around 200 – 250 °C, which was obviously lower than the densification temperature of LATP at conventional sintering in furnace. The increase of neck region at the grain-boundary of LATP particles could be also observed by cross-sectional SEM image of LATP pellet. As a result, total Li-ion conductivities for LATP pellet was 2.2 x 10-5 S cm-1 at 30 °C. Composite electrode powder was prepared from a mixture of 50 wt% carbon-coated LiFePO4 and 50 wt% LATP electrolyte. Au/composite electrode powder/ LATP powder was assembled by SPS process at the same condition for the sample of AC impedance measurement. Lithium foil was used as a reference/counter electrode. A poly(ethylene oxide)-based polymer electrolyte film was inserted between the lithium foil and the LATP electrolyte separator to prevent the reduction of Ti4+ ion in LATP by contacting with the lithium metal. Electrochemical charge-discharge test was performed at a constant current of 10 μA cm-2. The ASS-LIB shows initial charge-discharge profile which was similar to the liquid electrolyte case, and the discharge capacity was 75 mAh g-1 at 28 °C and 131 mAh g-1 at 60°C, respectively. No impurity peak was observed in the powder XRD pattern of the LiFePO4-LATP composite electrode after SPS process, due to the low sintering temperature. Therefore, the low-temperature and high-pressure SPS process with tungsten carbide die would be one of the superior techniques for assembling ASS-LIBs with suppressing the formation of by-products and reducing grain-boundary resistance at electrode/electrolyte and electrolyte/electrolyte interfaces.

Spark plasma sintering versus hot pressing – densification, bending strength, microstructure, and tribological properties of Ti5Al2.5Fe alloys

The influence of different sintering temperatures during Spark Plasma Sintering (SPS) process on microstructure of La-Ca-Sr-Mn-O ceramics has been studied. The powders of La0.67Ca0.33-xSrxMnO3 (x = 0.33; 0.03) (LCSM) perovskite were prepared by milling of the stoichiometric amounts of the starting materials - lanthanum oxide (La2O3), calcium oxide (CaO), strontium carbonate (SrCO3) and manganese oxide (MnO2), and subsequently calcinated twice. After the second calcinations the LCSM powders were treated by SPS method at four different temperatures (1000°C, 1150°C, 1200°C and 1250°C), at uniaxial pressure of 50 MPa in a vacuum. The microstructure characterizations were done by polarized light microscopy and scanning electron microscopy. The microstructural observations showed that increasing sintering temperature leads to an increase of grain size. The energy dispersive spectral (EDS) analysis confirmed that higher sintering temperatures cause changes in the phase composition of the investigated LCSM perovskite materials. The benefits of the LCSM samples preparation by SPS process are discussed.

Feasibility of producing porous gradient structure by spark plasma sintering (SPS) process was examined. Adequate combination of porosity gradient and pore size distribution could be obtained by appropriately controlling the SPS parameter such as sintering temperature, sintering time, applied pressure, and stopper length. For the longitudinal porous gradient structure, pure W sample was prepared by specially shaped graphite mold. Stainless steel powder was employed for the radially layered porous structure with different pore size. The graded porous structure could be applied for the fabrication of W-Cu FGM by Cu-infiltration and high temperature filter with high filtration efficiency.

Titanium matrix composites reinforced with TiB whiskers were in situ synthesized by spark plasma sintering (SPS) at a temperature range of 1173­1473K, using a mixture of 6.2mass% TiB2, 4.1mass% Ti, and 89.7mass% Ti6Al4V powders. The in-situ synthesis mechanism and the effect of sintering temperature on the sintering microstructure were investigated. The results show that an increase of the sintering temperature causes the relative densities of the composites to increase and the reaction between Ti and TiB2 to be more complete. Nano-sized TiB reinforcements with a diameter of around 80nm and fine-grained matrix with an average grain size of 12µm are obtained after SPSed at 1373K. Clean and perfect TiB/matrix interfaces without debonding or cracks are obtained during SPS process. Low sintering temperature and short sintering time are believed to be the main reasons for the improved fine-grained microstructure. [doi:10.2320/matertrans.M2014347]

Microstructure and transformation of Al-containing nanostructured 316L stainless steel coatings processed using spark plasma sintering